I have read the very helpful project page but worried that even when I turn off the water because I am working on hot tap upstairs that water will still come out as it is attached to a tank? will this be the case? we have soft water so assume that it will just need a washer change?

yes, tap will run for a while until water level in cylinder = tap level. Shutting off gate valve on cylinder feed from cold cistern will speed this up, or better still service valve near tap if you have one.